1. Field of the Invention
The present invention relates to a power supply unit of an electrifying unit used in an image forming process according to an electrophotographic method, and, in detail, to a power supply unit in which an erroneous output to the electrifying unit is mitigated, an image forming apparatus including the power supply unit, and a power supply control method that the power supply unit carries out.
2. Description of the Related Art
In the related art, in an image forming process of a roller electrifying method according to the electrophotographic method, an AC-DC superposing method in which an alternating current voltage is superposed on a direct current voltage is suitably used in order to achieve a uniform electrifying voltage on a surface of a roller of an electrifying unit. Further, recently, in order to achieve high amplifying efficiency, a class D amplifier is used instead of an analog amplifier (see Japanese Patent No. 3632428: patent document 1). A class D amplifier is employed also in an alternating current high-voltage power supply unit used for supplying a high-voltage alternating current bias to the electrifying unit. Thereby, amplifying efficiency improves, a heat generation amount is reduced, then power consumption of an image forming apparatus is reduced, and further, it is possible to avoid having a heat radiation plate that is necessary for the amplifier in a case of the analog amplifier.
However, in the alternating current high-voltage power supply unit employing the class D amplifier in the related art, an erroneous output may occur when the class D amplifier is used the first time immediately after a power supply voltage is turned on in a drive circuit of the alternating current high-voltage power supply unit. If the erroneous output has occurred, a discharge may occur in an electrifying unit depending on a magnitude of the erroneous output, a discharge history may remain on a photosensitive member, and thus, a color band or a white band may occur in an image formed by an image forming apparatus that employs the alternating current high-voltage power supply unit and the electrifying unit.